The new Aerospace Recommend Practice ARP4761A, just released in 2023, should continue to advance aviation safety into the next decade. The new independent https://arp4754a.org/ provides free guidance on ARP4754A, ARP4754B, and ARP4761A for all users.
ARP4761A advances an Aircraft-level Safety Assessment (ASA) which is a systematic, comprehensive evaluation of the implemented aircraft to verify that the implemented aircraft design meets the safety requirements as defined in the Preliminary Aircraft Safety Assessment (PASA). The ASA determines that the requirements from the AFHA and PASA have been met. The ASA also demonstrates that aircraft architecture, the relationships between aircraft functions and systems are acceptable. ARP4761A PDF technical whitepapers can be downloaded from the AFuzion website.
ASA is kind of final assessment that covers results and evidence of safety assessments performed during development process. The ASA basically covers;
- The list of Aircraft FHA Failure Condition with the evidence that they are satisfied.
- Evidence that Safety Program Plan objectives have been achieved.
- Evidence that aircraft architecture meets the qualitative and quantitative safety requirements.
- Evidence that aircraft architecture meets the Development Assurance Level allocation requirements.
- The status of open problem reports and their consequences on the aircraft.
Fault Tree Analysis (FTA) within ARP4761A is a top-down analysis technique to determine what single failures or combinations of failures can exist at the lower levels that might cause each failure condition. The primary purpose of ARP4761A’s FTA is to determine the probability of occurrence of the top event — therefore, demonstrating compliance with a probability requirement specified in a higher level document (usually an FHA).
Failure Modes and Effects Analysis (FMEA) within ARP4761A is a systematic, bottom-up analysis performed to identify the failure modes of a system, item, or function and determining the effect of the failure on the next higher level. FMEA’s can be done at the component, function, or LRU level. Generally, an FMEA deals with the individual and the combined effects of single failures.
The acceptance of adequate probability of failure conditions is often derived from the assessment of multiple systems based on the assumption that failures are independent. This independence might not exist in the practical sense, and specific studies are necessary to ensure that independence can either be assured or deemed acceptable. The CCA is concerned with events that could lead to a hazardous or catastrophic failure condition. The CCA is divided into three areas of study:
- Zonal Safety Analysis (ZSA) — The objective of this analysis is to ensure that the system and equipment installations within each zone of the aircraft are at an adequate safety standard regarding design and installation, interference between systems, and maintenance errors.
- Particular Risks Analysis (PRA) — Particular risks are those events or influences outside the systems of interest (for example, fire, leaking fluids, bird strike, HIRF, lightning, etc.). Each risk should be the subject of a specific study to examine and document the simultaneous or cascading effects (or influences) that might violate independence. The objective of the PRA is to ensure that the safety related effects are either eliminated or that the risk is acceptable.
- Common Mode Analysis (CMA) — The CMA is performed to confirm the assumed independence of the events that were considered in combination for a given failure condition. Another way of saying this is that the CMA is performed to verify that combinatorial events in the FTA are truly independent in the actual implementation. The effects of development, manufacturing, installation, maintenance and crew errors, and failures of system components that defeat the independence should be analyzed.
